getfsstat(2) MNT_NOWAIT & stale data for zpool

From: Dave Cottlehuber <dch_at_skunkwerks.at>
Date: Wed, 08 Oct 2025 15:59:12 UTC 
When does getfsstat(2) stale info get updated? It seems this can
be easily more than 15 minutes, based on experiments in prod. It
can be triggered by running `/bin/df` but not `/bin/df /`, which
would be preferable.

Background:

I'm using prometheus node_exporter on a zpool to monitor disk usage.
Right now this is broken, as a zpool can fill up, and node_exporter
continues to return the old data.

It loops over all filesystems, doing unix.Getfsstat(buf, unix.MNT_NOWAIT),
which is a wrapper around getfsstat(2) which clearly states:

     Normally mode should be specified as MNT_WAIT.  If mode is set to MNT_NOWAIT, getfsstat() will return the information it has
     available without requesting an update from each file system.  Thus, some of the information will be out of date, but getfsstat()
     will not block waiting for information from a file system that is unable to respond.  It will also skip any file system that is in
     the process of being unmounted, even if the unmount would eventually fail.

Is node_exporter just broken and should be fixed to use MNT_WAIT?

Is there some tunable to refresh this stale data more frequently, 
or should I just add `/bin/df` to a regular crontab?

A+
Dave